It has been hypothesized that intrauterine exposure to increased levels of endogenous pregnancy estrogens may play an important role in the etiology of breast cancer. There are several likely indicators of levels of pregnancy estrogens but for two of them the evidence is overwhelming and the respective relations strong: pregnancy toxemia (preeclampsia/eclampsia) is associated with low levels of pregnancy estrogens; and there is a positive correlation between pregnancy estrogens and measures of fetal size. The investigators propose to examine the relation between these indicators and subsequent breast cancer occurrence in a nested case-control study in Sweden. The base population consists of more than 150,000 women who were delivered at the Uppsala University Hospital from 1874 to 1954 and at the county hospitals of Sundsvall, Falun, and Eskilstuno from 1920 to 1857. Incident cases of breast cancer among these women will be identified through their unique national registration number in the Swedish cancer Registry during 1958-1990. For each case the females born to the first three mothers admitted after the case's mother will be selected as potential matching controls. Only controls living in Sweden and free from breast cancer until the time of diagnosis of breast cancer in the corresponding case will be included in the analysis. Tracing of eligible controls is done through successive parish data, the Swedish Death Registry, and the Swedish Cancer Registry. It is estimated that approximately 1,100 breast cancer cases and 3,100 age and birth-date matched controls will be included in the study. Standard analytic methods with conditional logistic regression for matched case-control studies will be carried out.Maternal length and placental weight will be examined as the risk determinants. In addition, maternal preeclampsia and eclampsia during the previous pregnancies will be analyzed to address whether features during the pregnancy or constitutional characteristics of the mother are the more important determinants.